Method and apparatus for transmitting data in CU-DU split scenario

ABSTRACT

Provided are a method in which a source distribution unit (DU) of a base station stops transmitting data to a user equipment (UE) in a wireless communication system, and an apparatus supporting the method. The method may include: receiving, from a central unit (CU) of the base station, a message indicating to stop transmitting data to the UE; and stopping transmitting data to the UE, upon receiving the message from the CU of the base station.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. patentapplication Ser. No. 16/064,832, filed Jun. 21, 2018, which is theNational Stage filing under 35 U.S.C. 371 of International ApplicationNo. PCT/KR2018/002680, filed on Mar. 7, 2018, which claims the benefitof U.S. Provisional Applications No. 62/467,830 filed on Mar. 7, 2017,No. 62/475,227 filed on Mar. 23, 2017, No. 62/488,076 filed on Apr. 21,2017, No. 62/536,468 filed on Jul. 25, 2017 and No. 62/555,050 filed onSep. 7, 2017, the contents of which are all hereby incorporated byreference herein in their entirety.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a wireless communication system, andmore particularly, to a method of transmitting data in a scenario inwhich a central unit and distributed unit of a base station are split,and an apparatus supporting the method.

Related Art

In order to meet the demand for wireless data traffic soring since the4th generation (4G) communication system came to the market, there areongoing efforts to develop enhanced 5th generation (5G) communicationsystems or pre-5G communication systems. For the reasons, the 5Gcommunication system or pre-5G communication system is called the beyond4G network communication system or post long-term evolution (LTE)system.

SUMMARY OF THE INVENTION

Meanwhile, in a case where a central unit (CU) and a distributed unit(DU) are split between a packet data convergence protocol (PDCP) layerand a radio link control (RLC) layer, when a user equipment (UE) changesthe DU due to mobility of the UE, lost data may occur in a source DU.Therefore, there is a need to propose a procedure for retransmitting thelost data.

According to an embodiment, there is provided a method in which a sourceDU of a base station stops transmitting data to a UE in a wirelesscommunication system. The method may include: receiving, from a CU ofthe base station, a message indicating to stop transmitting data to theUE; and stopping transmitting data to the UE upon receiving the messagefrom the CU of the base station.

According to another embodiment, there is provided a source DU of a basestation which stops transmitting data to a UE in a wirelesscommunication system. The source DU may include: a memory; atransceiver; and a processor operatively coupling the memory and thetransceiver, wherein the processor is configured to: control thetransceiver to receive, from a CU of the base station, a messageindicating to stop transmitting data to the UE; and control thetransceiver to stop transmitting data to the UE upon receiving themessage from the CU of the base station.

A data loss caused by mobility of a user equipment (UE) can beprevented.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows LTE system architecture.

FIG. 2 shows a control plane of a radio interface protocol of an LTEsystem.

FIG. 3 shows a user plane of a radio interface protocol of an LTEsystem.

FIG. 4 shows a structure of a 5G system.

FIG. 5 shows a wireless interface protocol of a 5G system for a userplane.

FIG. 6 shows a split-type gNB deployment (centralized deployment)scenario.

FIG. 7 shows a function split between a central unit and a distributedunit in a split-type gNB deployment scenario.

FIG. 8 shows a data loss which occurs in a source DU when a UE movesbetween adjacent DUs within the same CU.

FIG. 9 shows a procedure in which a source DU of a gNB stopstransmitting data to a UE according to an embodiment of the presentinvention.

FIG. 10 shows an example of a downlink data delivery status frameaccording to an embodiment of the present invention.

FIG. 11a and FIG. 11b show a DU change procedure between adjacent DUswithin the same CU according to an embodiment of the present invention.

FIG. 12 shows a DU change procedure between adjacent DUs within the sameCU according to an embodiment of the present invention.

FIG. 13a and FIG. 13b show a DU change procedure between adjacent DUswithin the same CU according to an embodiment of the present invention.

FIG. 14a and FIG. 14b show a DU change procedure between adjacent DUswithin the same CU according to an embodiment of the present invention.

FIG. 15 is a block diagram of a method in which a source DU of a basestation stops transmitting data to a UE according to an embodiment ofthe present invention.

FIG. 16 is a block diagram illustrating a wireless communication systemaccording to the embodiment of the present invention.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

The technology described below can be used in various wirelesscommunication systems such as code division multiple access (CDMA),frequency division multiple access (FDMA), time division multiple access(TDMA), orthogonal frequency division multiple access (OFDMA), singlecarrier frequency division multiple access (SC-FDMA), etc. The CDMA canbe implemented with a radio technology such as universal terrestrialradio access (UTRA) or CDMA-2000. The TDMA can be implemented with aradio technology such as global system for mobile communications(GSM)/general packet ratio service (GPRS)/enhanced data rate for GSMevolution (EDGE). The OFDMA can be implemented with a radio technologysuch as institute of electrical and electronics engineers (IEEE) 802.11(Wi-Fi), IEEE 802.16 (WiMAX), IEEE 802.20, evolved UTRA (E-UTRA), etc.IEEE 802.16m is evolved from IEEE 802.16e, and provides backwardcompatibility with a system based on the IEEE 802.16e. The UTRA is apart of a universal mobile telecommunication system (UMTS). 3rdgeneration partnership project (3GPP) long term evolution (LTE) is apart of an evolved UMTS (E-UMTS) using the E-UTRA. The 3GPP LTE uses theOFDMA in a downlink and uses the SC-FDMA in an uplink. LTE-advanced(LTE-A) is an evolution of the LTE. 5G is an evolution of the LTE-A.

For clarity, the following description will focus on LTE-A. However,technical features of the present invention are not limited thereto.

FIG. 1 shows LTE system architecture. The communication network iswidely deployed to provide a variety of communication services such asvoice over internet protocol (VoIP) through IMS and packet data.

Referring to FIG. 1, the LTE system architecture includes one or moreuser equipment (UE; 10), an evolved-UMTS terrestrial radio accessnetwork (E-UTRAN) and an evolved packet core (EPC). The UE 10 refers toa communication equipment carried by a user. The UE 10 may be fixed ormobile, and may be referred to as another terminology, such as a mobilestation (MS), a user terminal (UT), a subscriber station (SS), awireless device, etc.

The E-UTRAN includes one or more evolved node-B (eNB) 20, and aplurality of UEs may be located in one cell. The eNB 20 provides an endpoint of a control plane and a user plane to the UE 10. The eNB 20 isgenerally a fixed station that communicates with the UE 10 and may bereferred to as another terminology, such as a base station (BS), a basetransceiver system (BTS), an access point, etc. One eNB 20 may bedeployed per cell. There are one or more cells within the coverage ofthe eNB 20. A single cell is configured to have one of bandwidthsselected from 1.25, 2.5, 5, 10, and 20 MHz, etc., and provides downlinkor uplink transmission services to several UEs. In this case, differentcells can be configured to provide different bandwidths.

Hereinafter, a downlink (DL) denotes communication from the eNB 20 tothe UE 10, and an uplink (UL) denotes communication from the UE 10 tothe eNB 20. In the DL, a transmitter may be a part of the eNB 20, and areceiver may be a part of the UE 10. In the UL, the transmitter may be apart of the UE 10, and the receiver may be a part of the eNB 20.

The EPC includes a mobility management entity (MME) which is in chargeof control plane functions, and a system architecture evolution (SAE)gateway (S-GW) which is in charge of user plane functions. The MME/S-GW30 may be positioned at the end of the network and connected to anexternal network. The MME has UE access information or UE capabilityinformation, and such information may be primarily used in UE mobilitymanagement. The S-GW is a gateway of which an endpoint is an E-UTRAN.The MME/S-GW 30 provides an end point of a session and mobilitymanagement function for the UE 10. The EPC may further include a packetdata network (PDN) gateway (PDN-GW). The PDN-GW is a gateway of which anendpoint is a PDN.

The MME provides various functions including non-access stratum (NAS)signaling to eNBs 20, NAS signaling security, access stratum (AS)security control, Inter core network (CN) node signaling for mobilitybetween 3GPP access networks, idle mode UE reachability (includingcontrol and execution of paging retransmission), tracking area listmanagement (for UE in idle and active mode), P-GW and S-GW selection,MME selection for handovers with MME change, serving GPRS support node(SGSN) selection for handovers to 2G or 3G 3GPP access networks,roaming, authentication, bearer management functions including dedicatedbearer establishment, support for public warning system (PWS) (whichincludes earthquake and tsunami warning system (ETWS) and commercialmobile alert system (CMAS)) message transmission. The S-GW host providesassorted functions including per-user based packet filtering (by e.g.,deep packet inspection), lawful interception, UE Internet protocol (IP)address allocation, transport level packet marking in the DL, UL and DLservice level charging, gating and rate enforcement, DL rate enforcementbased on APN-AMBR. For clarity MME/S-GW 30 will be referred to hereinsimply as a “gateway,” but it is understood that this entity includesboth the MME and S-GW.

Interfaces for transmitting user traffic or control traffic may be used.The UE 10 and the eNB 20 are connected by means of a Uu interface. TheeNBs 20 are interconnected by means of an X2 interface. Neighboring eNBsmay have a meshed network structure that has the X2 interface. The eNBs20 are connected to the EPC by means of an S1 interface. The eNBs 20 areconnected to the MME by means of an S1-MME interface, and are connectedto the S-GW by means of S1-U interface. The S1 interface supports amany-to-many relation between the eNB 20 and the MME/S-GW.

The eNB 20 may perform functions of selection for gateway 30, routingtoward the gateway 30 during a radio resource control (RRC) activation,scheduling and transmitting of paging messages, scheduling andtransmitting of broadcast channel (BCH) information, dynamic allocationof resources to the UEs 10 in both UL and DL, configuration andprovisioning of eNB measurements, radio bearer control, radio admissioncontrol (RAC), and connection mobility control in LTE_ACTIVE state. Inthe EPC, and as noted above, gateway 30 may perform functions of pagingorigination, LTE_IDLE state management, ciphering of the user plane, SAEbearer control, and ciphering and integrity protection of NAS signaling.

FIG. 2 shows a control plane of a radio interface protocol of an LTEsystem. FIG. 3 shows a user plane of a radio interface protocol of anLTE system.

Layers of a radio interface protocol between the UE and the E-UTRAN maybe classified into a first layer (L1), a second layer (L2), and a thirdlayer (L3) based on the lower three layers of the open systeminterconnection (OSI) model that is well-known in the communicationsystem. The radio interface protocol between the UE and the E-UTRAN maybe horizontally divided into a physical layer, a data link layer, and anetwork layer, and may be vertically divided into a control plane(C-plane) which is a protocol stack for control signal transmission anda user plane (U-plane) which is a protocol stack for data informationtransmission. The layers of the radio interface protocol exist in pairsat the UE and the E-UTRAN, and are in charge of data transmission of theUu interface.

A physical (PHY) layer belongs to the L1. The PHY layer provides ahigher layer with an information transfer service through a physicalchannel. The PHY layer is connected to a medium access control (MAC)layer, which is a higher layer of the PHY layer, through a transportchannel. A physical channel is mapped to the transport channel Data istransferred between the MAC layer and the PHY layer through thetransport channel. Between different PHY layers, i.e., a PHY layer of atransmitter and a PHY layer of a receiver, data is transferred throughthe physical channel using radio resources. The physical channel ismodulated using an orthogonal frequency division multiplexing (OFDM)scheme, and utilizes time and frequency as a radio resource.

The PHY layer uses several physical control channels. A physicaldownlink control channel (PDCCH) reports to a UE about resourceallocation of a paging channel (PCH) and a downlink shared channel(DL-SCH), and hybrid automatic repeat request (HARQ) information relatedto the DL-SCH. The PDCCH may carry a UL grant for reporting to the UEabout resource allocation of UL transmission. A physical control formatindicator channel (PCFICH) reports the number of OFDM symbols used forPDCCHs to the UE, and is transmitted in every subframe. A physicalhybrid ARQ indicator channel (PHICH) carries an HARQ acknowledgement(ACK)/non-acknowledgement (NACK) signal in response to UL transmission.A physical uplink control channel (PUCCH) carries UL control informationsuch as HARQ ACK/NACK for DL transmission, scheduling request, and CQI.A physical uplink shared channel (PUSCH) carries a UL-uplink sharedchannel (SCH).

A physical channel consists of a plurality of subframes in time domainand a plurality of subcarriers in frequency domain. One subframeconsists of a plurality of symbols in the time domain. One subframeconsists of a plurality of resource blocks (RBs). One RB consists of aplurality of symbols and a plurality of subcarriers. In addition, eachsubframe may use specific subcarriers of specific symbols of acorresponding subframe for a PDCCH. For example, a first symbol of thesubframe may be used for the PDCCH. The PDCCH carries dynamic allocatedresources, such as a physical resource block (PRB) and modulation andcoding scheme (MCS). A transmission time interval (TTI) which is a unittime for data transmission may be equal to a length of one subframe. Thelength of one subframe may be 1 ms.

The transport channel is classified into a common transport channel anda dedicated transport channel according to whether the channel is sharedor not. A DL transport channel for transmitting data from the network tothe UE includes a broadcast channel (BCH) for transmitting systeminformation, a paging channel (PCH) for transmitting a paging message, aDL-SCH for transmitting user traffic or control signals, etc. The DL-SCHsupports HARQ, dynamic link adaptation by varying the modulation, codingand transmit power, and both dynamic and semi-static resourceallocation. The DL-SCH also may enable broadcast in the entire cell andthe use of beamforming. The system information carries one or moresystem information blocks. All system information blocks may betransmitted with the same periodicity. Traffic or control signals of amultimedia broadcast/multicast service (MBMS) may be transmitted throughthe DL-SCH or a multicast channel (MCH).

A UL transport channel for transmitting data from the UE to the networkincludes a random access channel (RACH) for transmitting an initialcontrol message, a UL-SCH for transmitting user traffic or controlsignals, etc. The UL-SCH supports HARQ and dynamic link adaptation byvarying the transmit power and potentially modulation and coding. TheUL-SCH also may enable the use of beamforming. The RACH is normally usedfor initial access to a cell.

A MAC layer belongs to the L2. The MAC layer provides services to aradio link control (RLC) layer, which is a higher layer of the MAClayer, via a logical channel. The MAC layer provides a function ofmapping multiple logical channels to multiple transport channels. TheMAC layer also provides a function of logical channel multiplexing bymapping multiple logical channels to a single transport channel A MACsublayer provides data transfer services on logical channels.

The logical channels are classified into control channels fortransferring control plane information and traffic channels fortransferring user plane information, according to a type of transmittedinformation. That is, a set of logical channel types is defined fordifferent data transfer services offered by the MAC layer. The logicalchannels are located above the transport channel, and are mapped to thetransport channels.

The control channels are used for transfer of control plane informationonly. The control channels provided by the MAC layer include a broadcastcontrol channel (BCCH), a paging control channel (PCCH), a commoncontrol channel (CCCH), a multicast control channel (MCCH) and adedicated control channel (DCCH). The BCCH is a downlink channel forbroadcasting system control information. The PCCH is a downlink channelthat transfers paging information and is used when the network does notknow the location cell of a UE. The CCCH is used by UEs having no RRCconnection with the network. The MCCH is a point-to-multipoint downlinkchannel used for transmitting MBMS control information from the networkto a UE. The DCCH is a point-to-point bi-directional channel used by UEshaving an RRC connection that transmits dedicated control informationbetween a UE and the network.

Traffic channels are used for the transfer of user plane informationonly. The traffic channels provided by the MAC layer include a dedicatedtraffic channel (DTCH) and a multicast traffic channel (MTCH). The DTCHis a point-to-point channel, dedicated to one UE for the transfer ofuser information and can exist in both uplink and downlink. The MTCH isa point-to-multipoint downlink channel for transmitting traffic datafrom the network to the UE.

Uplink connections between logical channels and transport channelsinclude the DCCH that can be mapped to the UL-SCH, the DTCH that can bemapped to the UL-SCH and the CCCH that can be mapped to the UL-SCH.Downlink connections between logical channels and transport channelsinclude the BCCH that can be mapped to the BCH or DL-SCH, the PCCH thatcan be mapped to the PCH, the DCCH that can be mapped to the DL-SCH, andthe DTCH that can be mapped to the DL-SCH, the MCCH that can be mappedto the MCH, and the MTCH that can be mapped to the MCH.

An RLC layer belongs to the L2. The RLC layer provides a function ofadjusting a size of data, so as to be suitable for a lower layer totransmit the data, by concatenating and segmenting the data receivedfrom an upper layer in a radio section. In addition, to ensure a varietyof quality of service (QoS) required by a radio bearer (RB), the RLClayer provides three operation modes, i.e., a transparent mode (TM), anunacknowledged mode (UM), and an acknowledged mode (AM). The AM RLCprovides a retransmission function through an automatic repeat request(ARQ) for reliable data transmission. Meanwhile, a function of the RLClayer may be implemented with a functional block inside the MAC layer.In this case, the RLC layer may not exist.

A packet data convergence protocol (PDCP) layer belongs to the L2. ThePDCP layer provides a function of header compression function thatreduces unnecessary control information such that data being transmittedby employing IP packets, such as IPv4 or IPv6, can be efficientlytransmitted over a radio interface that has a relatively smallbandwidth. The header compression increases transmission efficiency inthe radio section by transmitting only necessary information in a headerof the data. In addition, the PDCP layer provides a function ofsecurity. The function of security includes ciphering which preventsinspection of third parties, and integrity protection which preventsdata manipulation of third parties.

A radio resource control (RRC) layer belongs to the L3. The RLC layer islocated at the lowest portion of the L3, and is only defined in thecontrol plane. The RRC layer takes a role of controlling a radioresource between the UE and the network. For this, the UE and thenetwork exchange an RRC message through the RRC layer. The RRC layercontrols logical channels, transport channels, and physical channels inrelation to the configuration, reconfiguration, and release of RBs. AnRB is a logical path provided by the L1 and L2 for data delivery betweenthe UE and the network. That is, the RB signifies a service provided theL2 for data transmission between the UE and E-UTRAN. The configurationof the RB implies a process for specifying a radio protocol layer andchannel properties to provide a particular service and for determiningrespective detailed parameters and operations. The RB is classified intotwo types, i.e., a signaling RB (SRB) and a data RB (DRB). The SRB isused as a path for transmitting an RRC message in the control plane. TheDRB is used as a path for transmitting user data in the user plane.

A Non-Access Stratum (NAS) layer placed over the RRC layer performsfunctions, such as session management and mobility management.

Referring to FIG. 2, the RLC and MAC layers (terminated in the eNB onthe network side) may perform functions such as scheduling, automaticrepeat request (ARQ), and hybrid automatic repeat request (HARQ). TheRRC layer (terminated in the eNB on the network side) may performfunctions such as broadcasting, paging, RRC connection management, RBcontrol, mobility functions, and UE measurement reporting andcontrolling. The NAS control protocol (terminated in the MME of gatewayon the network side) may perform functions such as a SAE bearermanagement, authentication, LTE_IDLE mobility handling, pagingorigination in LTE_IDLE, and security control for the signaling betweenthe gateway and UE.

Referring to FIG. 3, the RLC and MAC layers (terminated in the eNB onthe network side) may perform the same functions for the control plane.The PDCP layer (terminated in the eNB on the network side) may performthe user plane functions such as header compression, integrityprotection, and ciphering.

Hereinafter, a 5G network structure is described.

FIG. 4 shows a structure of a 5G system.

In case of an evolved packet core (EPC) having a core network structureof the existing evolved packet system (EPS), a function, a referencepoint, a protocol, or the like is defined for each entity such as amobility management entity (MME), a serving gateway (S-GW), a packetdata network gateway (P-GW), or the like.

On the other hand, in case of a 5G core network (or a NextGen corenetwork), a function, a reference point, a protocol, or the like isdefined for each network function (NF). That is, in the 5G core network,the function, the reference point, the protocol, or the like is notdefined for each entity.

Referring to FIG. 4, the 5G system structure includes at least one UE10, a next generation-radio access network (NG-RAN), and a nextgeneration core (NGC).

The NG-RAN may include at least one gNB 40, and a plurality of UEs maybe present in one cell. The gNB 40 provides the UE with end points ofthe control plane and the user plane. The gNB 40 is generally a fixedstation that communicates with the UE 10 and may be referred to asanother terminology, such as a base station (BS), a base transceiversystem (BTS), an access point, or the like. One gNB 40 may be arrangedin every cell. At least one cell may be present in a coverage of the gNB40.

The NGC may include an access and mobility function (AMF) and a sessionmanagement function (SMF) which are responsible for a function of acontrol plane. The AMF may be responsible for a mobility managementfunction, and the SMF may be responsible for a session managementfunction. The NGC may include a user plane function (UPF) which isresponsible for a function of a user plane.

Interfaces for transmitting user traffic or control traffic may be used.The UE 10 and the gNB 40 may be connected by means of a Uu interface.The gNBs 40 may be interconnected by means of an X2 interface.Neighboring gNBs 40 may have a meshed network structure based on an Xninterface. The gNBs 40 may be connected to an NGC by means of an NGinterface. The gNBs 40 may be connected to an AMF by means of an NG-Cinterface, and may be connected to a UPF by means of an NG-U interface.The NG interface supports a many-to-many-relation between the gNB 40 andthe AMF/UPF 50.

A gNB host may perform functions such as functions for radio resourcemanagement, IP header compression and encryption of user data stream,selection of an AMF at UE attachment when no routing to an AMF can bedetermined from the information provided by the UE, routing of userplane data towards UPF(s), scheduling and transmission of pagingmessages (originated from the AMF), scheduling and transmission ofsystem broadcast information (originated from the AMF or O&M), ormeasurement and measurement reporting configuration for mobility andscheduling.

An access and mobility function (AMF) host may perform primary functionssuch as NAS signalling termination, NAS signalling security, AS securitycontrol, inter CN node signalling for mobility between 3GPP accessnetworks, idle mode UE reachability (including control and execution ofpaging retransmission), tracking area list management (for UE in idleand active mode), AMF selection for handovers with AMF change, accessauthentication, or access authorization including check of roamingrights.

A user plane function (UPF) host may perform primary functions such asanchor point for Intra-/inter-RAT mobility (when applicable), externalPDU session point of interconnect to data network, packet routing &forwarding, packet inspection and user plane part of policy ruleenforcement, traffic usage reporting, uplink classifier to supportrouting traffic flows to a data network, branching point to supportmulti-homed PDU session, QoS handling for user plane, e.g. packetfiltering, gating, UL/DL rate enforcement, uplink traffic verification(SDF to QoS flow mapping), transport level packet marking in the uplinkand downlink, or downlink packet buffering and downlink datanotification triggering.

A session management function (SMF) host may perform primary functionssuch as session management, UE IP address allocation and management,selection and control of UP function, configuring traffic steering atUPF to route traffic to proper destination, controlling part of policyenforcement and QoS, or downlink data notification.

FIG. 5 shows a wireless interface protocol of a 5G system for a userplane.

Referring to FIG. 5, the wireless interface protocol of the 5G systemfor the user plane may include a new layer called a service dataadaptation protocol (SDAP) in comparison with an LTE system. A primaryservice and function of the SDAP layer includes mapping between qualityof service (QoS) flow and a data radio bearer (DRB) and QoS flow ID(QFI) marking in both DL and UL packets. A single protocol entity of theSDAP may be configured for each individual PDU session, except for dualconnectivity (DC) for which two entities can be configured.

Hereinafter, a 5G RAN Deployment Scenario Will be Described.

A 5G RAN may be classified into a ‘non-centralized deployment’ scenario,a ‘co-sited deployment with E-UTRA’ scenario, and a ‘centralizeddeployment’ scenario according to a shape of deploying a function of aBS in a central unit and a distributed unit and according to whether itcoexists with a 4G BS. In this specification, the 5G RAN, a gNB, a nextgeneration node B, a new RAN, and a new radio BS (NR BS) may imply anewly defined BS for 5G.

FIG. 6 shows a split-type gNB deployment (centralized deployment)scenario.

Referring to FIG. 6, a gNB may be split into a central unit and adistributed unit. That is, the gNB may be operated by being split in alayered manner. The central unit may perform a function of upper layersof the gNB, and the distributed unit may perform a function of lowerlayers of the gNB.

FIG. 7 shows a function split between a central unit and a distributedunit in a split-type gNB deployment scenario.

Referring to FIG. 7, in case of an option 1, an RRC layer is in acentral unit, and an RLC layer, a MAC layer, a physical layer, and an RFare in a distributed unit. In case of an option 2, the RRC layer and thePDCP layer are in the central unit, and the RLC layer, the MAC layer,the physical layer, and the RF are in the distributed unit. In case ofan option 3, the RRC layer, the PDCP layer, and an upper RLC layer arein the central unit, and a lower RLC layer, the MAC layer, the physicallayer, and the RF are in the central unit. In case of an option 4, theRRC layer, the PDCP layer, and the RLC layer are in the central unit,and the MAC layer, the physical layer, and the RF are in the distributedunit. In case of an option 5, the RRC layer, the PDCP layer, the RLClayer, and an upper MAC layer are in the central unit, and a lower MAClayer, the physical layer, and the RF are in the distributed unit. Incase of an option 6, the RRC layer, the PDCP layer, the RLC layer, andthe MAC layer are in the central unit, and the physical layer and the RFare in the distributed unit. In case of an option 7, the RRC layer, thePDCP layer, the RLC layer, the MAC layer, and an upper physical layerare in the central unit, and a lower physical layer and the RF are inthe distributed unit. In case of an option 8, the RRC layer, the PDCPlayer, the RLC layer, the MAC layer, and the physical layer are in thecentral unit, and the RF is in the distributed unit.

Hereinafter, the central unit may be referred to as a CU, and thedistributed unit may be referred to as a DU in the presentspecification. The CU may be a logical node which hosts a radio resourcecontrol (RRC), service data adaptation protocol (SDAP), and packet dataconvergence protocol (PDCP) layers of the gNB. The DU may be a logicalnode which hosts radio link control (RLC), media access control (MAC),and physical (PHY) layers of the gNB. Alternatively, the CU may be alogical node which hosts RRC and PDCP layers of an en-gNB.

Meanwhile, if the CU and the DU are split as in the option 2 of FIG. 7,a lost RLC PDU may occur in a source DU when a UE changes the DU due toits mobility of the UE.

FIG. 8 shows a data loss which occurs in a source DU when a UE movesbetween adjacent DUs within the same CU.

Referring to FIG. 8, it is assumed that a DU 1 and a DU 2 are controlledby the same CU, and a UE moves from a region of the DU 1 to a region ofthe DU 2. Therefore, the DU 1 may be a source DU, and the DU 2 may be atarget DU. When the UE moves between adjacent DUs (e.g., from the DU 1to the DU 2) within the same CU, a lost RLC PDU may occur in a source DU(i.e., DU 1), and the lost RLC PDU may need to be retransmitted by atarget DU (i.e., DU 2). Therefore, in order to provide the lost RLC PDUto the UE, the target DU (e.g., DU 2) must know which RLC PDU is lost.However, since the source DU (i.e., DU 1) and the target DU (i.e., DU 2)are separated physically and there is no interface between them, thetarget DU (i.e., DU 2) cannot know which RLC PDU is lost in the sourceDU (i.e., DU 1). Therefore, the target DU (i.e., DU 2) cannot retransmitthe lost RLC PDU occurring in the source DU (i.e., DU 1) to the UE. Tosolve the aforementioned problem, there is a need to propose a procedurefor retransmitting the lost data. Hereinafter, a method ofretransmitting lost data and an apparatus supporting the method aredescribed according to an embodiment of the present invention.

FIG. 9 shows a procedure in which a source DU of a gNB stopstransmitting data to a UE according to an embodiment of the presentinvention.

Referring to FIG. 9, in step S910, a CU may determine to change a DU ofthe UE from a source DU to a target DU. The source DU and the target DUmay belong to the same CU.

In step S920, the CU may transmit to the source DU a message indicatingto stop transmitting data to the UE. The message may be a UE contextmodification request message or a new message. For example, the UEcontext modification request message transmitted by the CU to providethe change of UE context information to the DU may be defined by Table1.

TABLE 1 Assigned IE/Group Name Presence Criticality Criticality MessageType M YES reject gNB-CU UE F1AP ID M YES reject gNB-DU UE F1AP ID M YESreject PSCell ID O YES Ignore DRX Cycle O YES ignore CU to DU RRCInformation O YES reject Transmission Stop Indicator O YES ignore

Referring to Table 1, the UE context modification request message mayinclude a Transmission Stop Indicator IE, and the Transmission StopIndicator IE may instruct the DU to stop transmitting data to the UE.

In step S930, the source DU may stop transmitting data to the UE. Inaddition, the source DU may transmit, to the CU, information regardingunsuccessfully transmitted downlink data to the UE. The information maybe included in a downlink data delivery status frame. For example, theinformation regarding the downlink data may be sequence numbers of PDCPPDU corresponding to lost RLC PDUs. For example, the informationregarding the downlink data may be the highest PDCP PDU sequence numbersuccessfully delivered in sequence to the UE among those PDCP PDUsreceived from the CU.

FIG. 10 shows an example of a downlink data delivery status frameaccording to an embodiment of the present invention.

Referring to FIG. 10, the downlink data delivery status frame mayinclude the highest NR PDCP PDU sequence number successfully deliveredin sequence to the UE among those NR PDCP PDUs received from the nodehosting the NR PDCP entity. The node hosting the NR PDCP entity may be aCU.

Returning to FIG. 9, in step S940, the CU may transmit to the target DUthe unsuccessfully transmitted downlink data to the UE. The downlinkdata may be a downlink packet, and the downlink packet may include anunsuccessfully transmitted PDCP PDU to the UE from the source DU.

According to an embodiment of the present invention, when the UE movesfrom the source DU to the target DU, the unsuccessfully transmitteddownlink data to the UE from the source DU may be transmitted quicklyfrom the CU to the target DU.

FIG. 11a and FIG. 11b show a DU change procedure between adjacent DUswithin the same CU according to an embodiment of the present invention.

Referring to FIG. 11a , in step S1100, a UE may enter an RRC_CONNECTEDstate.

In step S1101, a measurement report message may be triggered andtransmitted to a source DU.

In step S1102, the source DU may transmit to a CU a message including acontainer which piggybacks the measurement report message. The messagemay be an uplink RRC transport message or a new message.

In step S1103, the source DU may provide the CU with a feedback for thedownlink data toward the UE.

In step S1104, upon receiving the measurement report message and thefeedback from the source DU, the CU may determine to change a DU of theUE.

In step S1105 a, the CU may transmit to the source DU the messageindicating to stop transmitting data to the UE. The message may be a UEcontext modification request message, a DU change indication message, ora new message. In addition, the message may indicate that the source DUof the UE is changed. If the message must be transmitted after stepS1108, steps S1105 a and S1105 b may be skipped.

In step S1105 b, upon receiving the message from the CU, the source DUmay stop transmitting the data to the UE and transmit a downlink datadelivery status frame to the CU. The downlink data delivery status framemay be transmitted to inform the CU of unsuccessfully transmitteddownlink data to the UE. That is, the source DU may provide the CU witha feedback for the downlink data toward the UE including the informationon lost PDUs. For example, the information on lost PDUs may be sequencenumbers of PDCP PDU corresponding to lost RLC PDUs. For example, theinformation on lost PDUs may be the highest PDCP PDU sequence numbersuccessfully delivered in sequence to the UE among those PDCP PDUsreceived from the CU. Therefore, the CU may know the unsuccessfullytransmitted downlink data to the UE (e.g., a downlink packet includingan unsuccessfully transmitted PDCP PDU to the source DU), and thedownlink data may be transmitted from the CU to the target DU.

In step S1106, the CU may initiate the change of the DU by requestingthe target DU to allocate a radio resource and/or create UE context forthe UE. The change of the DU may be requested by a UE context setupprocedure or a bearer setup procedure. That is, the CU may transmit a UEcontext setup request message or a bearer setup request message to thetarget DU. The CU may include the followings per bearer in the bearersetup request message or the UE context setup request message.

-   -   RB ID (e.g., SRB or DRB ID)    -   TNL address for the CU    -   Uplink tunnel endpoint identifier (TEID) for the CU    -   RLC configuration    -   Logical channel configuration

In addition, the CU may include the followings in the bearer setuprequest message or the UE context setup request message.

-   -   CU UE FlAP ID    -   RRC context

The RRC context may contain information related to beam measurement,RSRQ, RSRP, RACH configuration, and/or RACH resource for the UE. All ora part of information may not be contained in the RRC context. Inaddition, when the CU receives the SgNB modification request messagefrom the MeNB in an EN-DC case, the bearer setup request or the UEcontext setup request message may include RLC/MAC/PHY layer relatedinformation and/or radio resource configuration among informationincluded in the RRC container provided by the MeNB. In addition, the UEcontext setup request message or the bearer setup request message maycontain an indication to inform the target DU of either an inter-DUmobility case or the EN-DC case.

In step S1107, when the target DU receives the request message from theCU, the target DU may establish UE context and/or bearers requested forthe UE, and may allocate a required resource on an F1 interface for thebearer requested to be established. In addition, for the UE, the targetDU may allocate an RACH and beam related to the resource and may set anRACH configuration, on the basis of received information included or notincluded in RRC context.

In step S1108, to indicate that the requested bearer and/or UE contextare established, the target DU may respond to the CU with a UE contextsetup response message or a bearer setup response message. The target DUmay include the followings per bearer in the UE context setup responsemessage or the bearer setup response message.

-   -   RB ID (e.g., SRB or DRB ID)    -   TNL address for the target DU    -   Downlink TEID for the target DU

In addition, the target DU may contain the followings in the bearersetup response or the UE context setup response message.

-   -   RRC context

The RRC context may include information related to an allocated RACHresource, a set RACH configuration, and an allocated beam. All or a partof information may not be contained in the RRC context. Upon receivingthe information included in the RRC container provided by the MeNBand/or an indication which indicates the EN-DC case, the target DU mayinclude the radio resource related information corresponding to onesreceived or corresponding to the EN-DC case.

In step S1109 a, if the CU receives the response message from the targetDU and if steps 1105 a and S1105 b are skipped, the CU may transmit tothe source DU a message indicating to stop transmitting data to the UE.The message may be a UE context modification request message, a DUchange indication message, or a new message. In addition, the messagemay indicate that the source DU of the message is changed.

If the steps S1105 a and S1105 b are not skipped, the steps S1109 a andS1109 b may be skipped, and the CU may retransmit a PDCP PDU related toa lost RLC PDU for each bearer to the target DU on the basis of afeedback provided in step S1105 b.

The CU may generate an RRC message to be provided to the UE. Forexample, the RRC message may be an RRC connection reconfigurationmessage. The RRC message may include RRC context received from thetarget DU.

In step S1109 b, upon receiving the message from the CU, the source DUmay stop transmitting the data to the UE and provide the CU with adownlink data delivery status frame. The downlink data delivery statusframe may be transmitted to inform the CU of unsuccessfully transmitteddownlink data to the UE. That is, the source DU may provide the CU witha feedback for downlink data towards the UE including information onlost PDUs. For example, the information on lost PDUs may be sequencenumbers of PDCP PDU corresponding to lost RLC PDUs. For example, theinformation on lost PDUs may be the highest PDCP PDU sequence numbersuccessfully delivered in sequence to the UE among those PDCP PDUsreceived from the CU. Thereafter, the CU may retransmit to the target DUa PDCP PDU related to the lost RLC PDU for each bearer on the basis ofthe feedback. That is, the CU may know unsuccessfully transmitteddownlink data to the UE (e.g., a downlink packet including anunsuccessfully transmitted PDCP PDU from the source DU), and thedownlink data may be transmitted from the DU to the target DU.

In step S1110, the CU may transmit to the source DU a message includinga container which piggybacks the RRC connection reconfiguration message.The message may be a downlink RRC transport message or a new message. Ifsteps S1105 a and S1106 b are skipped or if steps S1109 a and S1109 bare skipped, an indication to stop transmitting data to the UE andprovide the CU with a feedback for the downlink data toward the UE maybe contained in this message.

In step S1111, upon receiving the message from the CU, the source DU maytransmit the RRC connection reconfiguration message to the UE. If thesource DU receives an indication from the CU in step S1110, the sourceDU may stop transmitting data to the UE and may provide the CU with thefeedback for downlink data toward the UE. The feedback may include theinformation on lost PDUs. For example, the information on lost PDUs maybe sequence numbers of PDCP PDU corresponding to lost RLC PDUs. Forexample, the information on lost PDUs may be the highest PDCP PDUsequence number successfully delivered in sequence to the UE among thosePDCP PDUs received from the CU. Thereafter, the CU may retransmit to thetarget DU a PDCP PDU related to the lost RLC PDU for each bearer on thebasis of the feedback. That is, the CU may know unsuccessfullytransmitted downlink data to the UE (e.g., a downlink packet includingan unsuccessfully transmitted PDCP PDU from the source DU), and thedownlink data may be transmitted from the DU to the target DU.

Referring to FIG. 11b , in step S1112, the UE may disconnect theconnection with the source DU.

In step S1113, the UE may transmit the RRC connection reconfigurationcomplete message to the target DU.

In step S1114, the target DU may transmit to the CU a message includinga container which piggybacks the RRC connection reconfiguration completemessage. The message may be an uplink RRC transport message or a newmessage.

In step S1115, if the CU receives the message from the target DU, the CUmay trigger a UE context release procedure or bearer release proceduretowards the UE in order to release a radio resource and/or UE contextfor the UE.

According to an embodiment of the present invention, the CU may instructthe source DU having the F1 connection to stop transmitting downlinkdata before the CU triggers the RRC connection reconfigurationprocedure. In addition, the source DU may provide a feedback fordownlink data. Therefore, when the UE moves from the source DU to thetarget DU, unsuccessfully transmitted downlink data to the UE from thesource DU (e.g., a PDCP PDU related to lost RLC PDUs occurring in thesource DU) may be retransmitted quickly from the CU to the target DU. Inaddition, signalling for DU change may be reduced or minimizedTherefore, this invention can make the UE's experience better (e.g.smooth and seamless DU change or handover) and facilitate a RAN node tohandle data packets better during the DU change or the handover.

FIG. 12 shows a DU change procedure between adjacent DUs within the sameCU according to an embodiment of the present invention.

Referring to FIG. 12, in step S1200, a UE may enter an RRC_CONNECTEDstate.

In step S1201, the CU may initiate the change of the DU by requestingthe target DU to allocate a radio resource for the UE. The change of theDU may be requested by a bearer setup procedure. That is, the CU maytransmit a bearer setup request message to the target DU. The CU mayinclude the followings per bearer in the bearer setup request message.

-   -   RB ID (e.g., SRB or DRB ID)    -   TNL address for the CU    -   Uplink TEID for the CU    -   RLC configuration    -   Logical channel configuration

In step S1202, when the target DU receives the request message from theCU, the target DU may establish bearers requested for the UE, and mayallocate a required resource on an F1 interface for the bearer requestedto be established.

In step S1203, to indicate that the requested bearer is established, thetarget DU may respond to the CU with a UE context setup response messageor a bearer setup response message. The target DU may include thefollowings per bearer in the bearer setup response message.

-   -   RB ID (e.g., SRB or DRB ID)    -   TNL address for the target DU    -   Downlink TEID for the target DU

In step S1204, if the CU receives a response message from the target DU,the CU may transmit the RRC connection reconfiguration message includingnew configuration for accessing the target DU. The RRC connectionreconfiguration message may be transmitted to the UE via the source DU.

In step S1205, the UE may transmit the RRC connection reconfigurationcomplete message to the CU via the target DU.

In step S1206, if the CU receives the complete message from the UE, theCU may trigger the bearer release procedure by requesting the source DUto release a radio resource for the UE.

In step S1207, if the source DU receives a bearer release requestmessage, the source DU may release all of bearers for the UE and thecorresponding resources on an F1 interface.

In step S1208, the source DU may transmit to the CU a message includinginformation on lost PDUs. The message may be a lost PDU indicationmessage or a bearer release response message. The information on thelost PDU may be provided per bearer. For example, the information onlost PDUs may be sequence numbers of PDCP PDU corresponding to lost RLCPDUs. For example, the information on lost PDUs may be the highest PDCPPDU sequence number successfully delivered in sequence to the UE amongthose PDCP PDUs received from the CU.

In step S1209, if the CU receives the message from the source DU, the CUmay provide the target DU with the lost PDCP PDU on the basis of theinformation on lost PDUs contained in the received message.

According to an embodiment of the present invention, the source DU mayinform the CU, which has the F1 connection, of information on the lostPDCP PDU corresponding to the lost RLC PDU for a specific UE when thereis a change in DUs in the same CU due to UE's mobility. Therefore, whenthe UE moves from the source DU to the target DU, unsuccessfullytransmitted downlink data to the UE from the source DU (e.g., a PDCP PDUrelated to lost RLC PDUs occurring in the source DU) may beretransmitted quickly from the CU to the target DU. Therefore, thisinvention can make the UE's experience better (e.g. smooth and seamlessDU change or handover) and facilitate a RAN node to handle data packetsbetter during the DU change or the handover.

FIG. 13a and FIG. 13b show a DU change procedure between adjacent DUswithin the same CU according to an embodiment of the present invention.

Referring to FIG. 13a , in step S1300, a UE may enter an RRC_CONNECTEDstate.

In step S1301, a measurement report message may be triggered andtransmitted to a source DU via a source DU. In an F1 interface, themeasurement report message may be transmitted by using a containerincluded in an uplink RRC transport message or a new message.

In step S1302, the CU may determine to change a DU of the UE on thebasis of the measurement report message.

In step S1303 a-1, the CU may transmit to the source DU a DU statusreporting request message or a new message. The message may include abearer ID (e.g., a radio bearer ID) and a lost PDU reporting indication.The message may be transmitted to request for reporting a lost PDU tothe CU per bearer provided to the UE which performs the DU change whenthe RLC PDU is lost.

In step S1303 a-2, if the source DU receives a request message from theCU, the source DU may report when the RLC PDU transmitted to the UE islost for each bearer.

In step S1303 b, the CU may transmit to the source DU a lost PDUreporting control message or a new message. The message may include abearer ID (e.g., a radio bearer ID). The message may be transmitted torequest for reporting a lost PDU to the CU when the RLC PDU is lost. Ifthe source DU receives a request message from the CU, the source DU mayreport when the RLC PDU transmitted to the UE is lost for each bearer.

In step S1304, the CU may initiate the change of the DU by requestingthe target DU to allocate a radio resource and/or create UE context forthe UE. The change of the DU may be requested by a UE context setupprocedure or a bearer setup procedure. That is, the CU may transmit a UEcontext setup request message or a bearer setup request message to thetarget DU. The CU may include the followings per bearer in the bearersetup request message or the UE context setup request message.

-   -   RB ID (e.g., SRB or DRB ID)    -   TNL address for the CU    -   Uplink TEID for the CU    -   RLC configuration    -   Logical channel configuration

In addition, the CU may include the followings in the bearer setuprequest message or the UE context setup request message.

-   -   CU UE FIAP ID    -   RRC context

The RRC context may contain information related to beam measurement,RSRQ, RSRP, RACH configuration, and/or RACH resource for the UE. All ora part of information may not be contained in the RRC context.

In step S1305, when the target DU receives the request message from theCU, the target DU may establish UE context and/or bearers requested forthe UE, and may allocate a required resource on an F1 interface for thebearer requested to be established. In addition, for the UE, the targetDU may allocate an RACH and beam related to the resource and may set anRACH configuration, on the basis of received information included or notincluded in RRC context.

In step S1306, to indicate that the requested bearer and/or UE contextare established, the target DU may respond to the CU with a UE contextsetup response message or a bearer setup response message. The target DUmay include the followings per bearer in the UE context setup responsemessage or the bearer setup response message.

-   -   RB ID (e.g., SRB or DRB ID)    -   TNL address for the target DU    -   Downlink TEID for the target DU

In addition, the target DU may contain the followings in the bearersetup response or the UE context setup response message.

-   -   RRC context

The RRC context may include information related to an allocated RACHresource, a set RACH configuration, and an allocated beam. All or a partof information may not be contained in the RRC context.

In step S1307 a, when the RLC PDU is lost for the bearer after stepS1303 a-2, the source DU may transmit to the CU a new message or a DUstatus update message including a bearer ID (e.g., a radio bearer ID)and information on lost PDUs. For example, the information on lost PDUsmay be sequence numbers of PDCP PDU corresponding to lost RLC PDUs. Forexample, the information on lost PDUs may be the highest PDCP PDUsequence number successfully delivered in sequence to the UE among thosePDCP PDUs received from the CU. The information on the lost PDUs may beprovided per bearer. Step S1307 a may be performed between step S1304and step S1306.

In step S1307 b, when the RLC PDU is lost for the bearer after stepS1303 b, the source DU may transmit to the CU a new message or a lostPDU report message including a bearer ID (e.g., a radio bearer ID) andinformation on lost PDUs. For example, the information on lost PDUs maybe sequence numbers of PDCP PDU corresponding to lost RLC PDUs. Forexample, the information on lost PDUs may be the highest PDCP PDUsequence number successfully delivered in sequence to the UE among thosePDCP PDUs received from the CU. The information on the lost PDUs may beprovided per bearer. Step S1307 b may be performed between step S1304and step S1306.

In step S1308, if the CU receives the bearer setup response or the UEcontext setup response message from the target DU, the CU may transmitthe RRC connection reconfiguration message including a new configuration(e.g., RRC context received from the target DU) for accessing the targetDU. The RRC connection reconfiguration message may be transmitted to theUE via the source DU. The RRC connection reconfiguration message may betransmitted using a container included in a downlink RRC transport or anew message.

Referring to FIG. 13b , in step S1309, the UE may disconnect theconnection with the source DU.

In step S1310, the UE may transmit the RRC connection reconfigurationcomplete message to the CU via the target DU. The RRC connectionreconfiguration complete message may be transmitted using a containerincluded in an uplink RRC transport or a new message.

In step S1311, if the CU receives the RRC connection reconfigurationcomplete message from the UE, the CU may retransmit to the target DU aPDCP PDU related to the lost RLC PDU for each bearer on the basis of abearer ID and information on the lost PDU per bearer received in stepS1307 a or S1307 b.

In step S1312, if the CU receives the RRC connection reconfigurationcomplete message from the UE, the CU may trigger a bearer releaseprocedure or UE context release procedure by requesting the source DU torelease a radio resource and/or UE context for the UE.

In step S1313, upon receiving a bearer release request message or a UEcontext release request message, the source DU may release all ofbearers and/or UE context for the UE and corresponding resources on anF1 interface.

In step S1314, the source DU may transmit a bearer release response or aUE context release response message to the CU.

According to an embodiment of the present invention, the source DU mayinform the CU, which has the F1 connection, of information on the lostPDCP PDU corresponding to the lost RLC PDU for a specific UE when thereis a change in DUs in the same CU due to UE's mobility. Therefore, whenthe UE moves from the source DU to the target DU, unsuccessfullytransmitted downlink data to the UE from the source DU (e.g., a PDCP PDUrelated to lost RLC PDUs occurring in the source DU) may beretransmitted quickly from the CU to the target DU. Therefore, thisinvention can make the UE's experience better (e.g. smooth and seamlessDU change or handover) and facilitate a RAN node to handle data packetsbetter during the DU change or the handover.

FIG. 14a and FIG. 14b show a DU change procedure between adjacent DUswithin the same CU according to an embodiment of the present invention.

Referring to FIG. 14a , in step S1400, a UE may enter an RRC_CONNECTEDstate.

In step S1401, a measurement report message may be triggered andtransmitted to a source DU via a source DU. In an F1 interface, themeasurement report message may be transmitted by using a containerincluded in an uplink RRC transport message or a new message.

In step S1402, the CU may determine to change a DU of the UE on thebasis of the measurement report message.

In step S1403, the CU may initiate the change of the DU by requestingthe target DU to allocate a radio resource and/or create UE context forthe UE. The change of the DU may be requested by a UE context setupprocedure or a bearer setup procedure. That is, the CU may transmit a UEcontext setup request message or a bearer setup request message to thetarget DU. The CU may include the followings per bearer in the bearersetup request message or the UE context setup request message.

-   -   RB ID (e.g., SRB or DRB ID)    -   TNL address for the CU    -   Uplink TEID for the CU    -   RLC configuration    -   Logical channel configuration

In addition, the CU may include the followings in the bearer setuprequest message or the UE context setup request message.

-   -   CU UE FIAP ID    -   RRC context

The RRC context may contain information related to beam measurement,RSRQ, RSRP, RACH configuration, and/or RACH resource for the UE. All ora part of information may not be contained in the RRC context.

In step S1404, when the target DU receives the request message from theCU, the target DU may establish UE context and/or bearers requested forthe UE, and may allocate a required resource on an F1 interface for thebearer requested to be established. In addition, for the UE, the targetDU may allocate an RACH and beam related to the resource and may set anRACH configuration, on the basis of received RRC context.

In step S1405, to indicate that the requested bearer and/or UE contextare established, the target DU may respond to the CU with UE contextsetup response message or a bearer setup response message. The target DUmay include the followings per bearer in the UE context setup responsemessage or the bearer setup response message.

-   -   RB ID (e.g., SRB or DRB ID)    -   TNL address for the target DU    -   Downlink TEID for the target DU

In addition, the target DU may contain the followings in the bearersetup response or the UE context setup response message.

-   -   RRC context

The RRC context may include information related to an allocated RACHresource, a set RACH configuration, and an allocated beam. All or a partof information may not be contained in the RRC context.

In step S1406, if the CU receives a response message from the target DU,the CU may transmit a UE context release request message or a bearerrelease request to the source DU to release a radio resource and/or UEcontext for the UE.

In step S1407, upon receiving the UE context release request message orthe bearer release request message, the source DU may transmit to the CUa message including information on a lost PDU and a bearer ID. Themessage may be a lost PDU indication message, a bearer release responsemessage, or a UE context release response message. The information onthe lost PDU and the bearer ID may be provided per bearer. For example,the information for the bearer ID may be a radio bearer ID. For example,the information on the lost PDU may be sequence numbers of PDCP PDUcorresponding to lost RLC PDUs. For example, the information on the lostPDU may be the highest PDCP PDU sequence number successfully deliveredin sequence to the UE among those PDCP PDUs received from the CU. Inaddition, the source DU may release all of bearers and/or UE context forthe UE and corresponding resources on an F1 interface.

In step S1408, if the CU receives a message from the source DU, the CUmay transmit an RRC connection reconfiguration message including a newconfiguration (e.g., RRC context received from the target DU) foraccessing the target DU. The RRC connection reconfiguration message maybe transmitted to the UE via the source DU. The RRC connectionreconfiguration message may be transmitted using a container included ina downlink RRC transport or a new message.

Referring to FIG. 14b , in step S1409, the UE may disconnect theconnection with the source DU.

In step S1410, the UE may transmit the RRC connection reconfigurationcomplete message to the CU via the target DU. The RRC connectionreconfiguration complete message may be transmitted using a containerincluded in an uplink RRC transport or a new message.

In step S1411, if the CU receives the RRC connection reconfigurationcomplete message from the UE, the CU may retransmit to the target DU aPDCP PDU related to the lost RLC PDU for each bearer on the basis of abearer ID and information on the lost PDU per bearer received in stepS1407.

According to an embodiment of the present invention, the source DU mayinform the CU, which has the F1 connection, of information on the lostPDCP PDU corresponding to the lost RLC PDU for a specific UE when thereis a change in DUs in the same CU due to UE's mobility, before the CUtriggers the RRC connection reconfiguration procedure. Therefore, whenthe UE moves from the source DU to the target DU, unsuccessfullytransmitted downlink data to the UE from the source DU (e.g., a PDCP PDUrelated to lost RLC PDUs occurring in the source DU) may beretransmitted quickly from the CU to the target DU. In addition,signalling for DU change may be reduced or minimized. Therefore, thisinvention can make the UE's experience better (e.g. smooth and seamlessDU change or handover) and facilitate a RAN node to handle data packetsbetter during the DU change or the handover.

FIG. 15 is a block diagram of a method in which a source DU of a basestation stops transmitting data to a UE according to an embodiment ofthe present invention.

Referring to FIG. 15, in step S1510, the source DU of the base stationmay receive from a central unit (CU) a message indicating to stoptransmitting data to the UE. The CU may be a logical node which hosts aradio resource control (RRC), service data adaptation protocol (SDAP),and packet data convergence protocol (PDCP) layer of the base station,and the DU may be a logical node which hosts a radio link control (RLC),media access control (MAC), and physical (PHY) layer of the basestation. The message may be a UE context modification request message.

In step S1520, upon receiving the message from the CU of the basestation, the source DU of the base station may stop transmitting data tothe UE.

In addition, the source DU of the base station may transmit, to the CUof the base station, information on unsuccessfully transmitted downlinkdata to the UE from the source DU of the base station. The informationon the downlink data may be information on a lost protocol data unit(PDU). The information on the downlink data may be the highest PDCP PDUsequence number successfully delivered in sequence to the UE among thosePDCP PDUs received from the CU of the base station. The information onthe downlink data may be a PDCP PDU sequence number corresponding to thelost RLC PDU. The information on the downlink data may be included in adownlink data delivery status frame. The downlink data may beretransmitted to a target DU of the base station with respect to eachbearer by the CU of the bae station on the basis of the information onthe downlink data.

In addition, the source DU of the base station may receive a measurementreport message from the UE. Further, the source DU of the base stationmay transmit an uplink RRC transport message including the measurementreport message to the CU of the base station. If the CU of the basestation determines to change a source DU of the base station for the UEon the basis of the measurement report message included in the uplinkRRC transport message, the message indicating to stop transmitting datato the UE may be received from the CU of the base station.

In addition, the source DU of the base station may transmit an RRCconnection reconfiguration message to the UE, upon receiving the messagefrom the CU of the base station.

FIG. 16 is a block diagram illustrating a wireless communication systemaccording to the embodiment of the present invention.

A UE 1600 includes a processor 1601, a memory 1602 and a transceiver1603. The memory 1602 is connected to the processor 1601, and storesvarious information for driving the processor 1601. The transceiver 1603is connected to the processor 1601, and transmits and/or receives radiosignals. The processor 1601 implements proposed functions, processesand/or methods. In the above embodiment, an operation of the userequipment may be implemented by the processor 1601.

A DU of a base station 1610 includes a processor 1611, a memory 1612 anda transceiver 1613. The memory 1612 is connected to the processor 1611,and stores various information for driving the processor 1611. Thetransceiver 1613 is connected to the processor 1611, and transmitsand/or receives radio signals. The processor 1611 implements proposedfunctions, processes and/or methods. In the above embodiment, anoperation of the DU may be implemented by the processor 1611.

A CU of the base station 1620 includes a processor 1612, a memory 1622and a transceiver 1623. The memory 1622 is connected to the processor1621, and stores various information for driving the processor 1621. Thetransceiver 1623 is connected to the processor 1621, and transmitsand/or receives radio signals. The processor 1621 implements proposedfunctions, processes and/or methods. In the above embodiment, anoperation of the CU may be implemented by the processor 1621.

The processor may include an application-specific integrated circuit(ASIC), a separate chipset, a logic circuit, and/or a data processingunit. The memory may include a read-only memory (ROM), a random accessmemory (RAM), a flash memory, a memory card, a storage medium, and/orother equivalent storage devices. The transceiver may include abase-band circuit for processing a wireless signal. When the embodimentis implemented in software, the aforementioned methods can beimplemented with a module (i.e., process, function, etc.) for performingthe aforementioned functions. The module may be stored in the memory andmay be performed by the processor. The memory may be located inside oroutside the processor, and may be coupled to the processor by usingvarious well-known means.

Various methods based on the present specification have been describedby referring to drawings and reference numerals given in the drawings onthe basis of the aforementioned examples. Although each method describesmultiple steps or blocks in a specific order for convenience ofexplanation, the invention disclosed in the claims is not limited to theorder of the steps or blocks, and each step or block can be implementedin a different order, or can be performed simultaneously with othersteps or blocks. In addition, those ordinarily skilled in the art canknow that the invention is not limited to each of the steps or blocks,and at least one different step can be added or deleted withoutdeparting from the scope and spirit of the invention.

The aforementioned embodiment includes various examples. It should benoted that those ordinarily skilled in the art know that all possiblecombinations of examples cannot be explained, and also know that variouscombinations can be derived from the technique of the presentspecification. Therefore, the protection scope of the invention shouldbe determined by combining various examples described in the detailedexplanation, without departing from the scope of the following claims.

What is claimed is:
 1. A method performed by a central unit (CU) of abase station (BS) configured to operate in a wireless communicationsystem, the method comprising: receiving, from a source distributed unit(DU) of the BS, a container including a measurement report transmittedby a user equipment (UE); initiating mobility for the UE from the sourceDU to a target DU of the BS within the CU; transmitting, to the targetDU, a UE context setup request message to create a UE context for the UEand allocate radio resources; receiving, from the target DU, a UEcontext response message in response to the UE context setup requestmessage; generating a radio resource control (RRC) reconfigurationmessage to be provided to the UE; transmitting, to the source DU, amessage including information related to stopping data transmission tothe UE and the generated RRC reconfiguration message, wherein, after thesource DU receives the message, data transmission to the UE is stoppedand the RRC reconfiguration message is provided to the UE; andreceiving, from the source DU, information related to downlink dataunsuccessfully transmitted to the UE, wherein RRC, packet dataconvergence protocol (PDCP) layers of the BS are located in the CU, andwherein radio link control (RLC), media access control (MAC), andphysical layers of the BS are located in the source DU and the targetDU.
 2. The method of claim 1, wherein the message transmitted to thesource DU is a UE context modification request message.
 3. The method ofclaim 1, wherein the information related to the downlink data isreceived via a downlink data delivery status frame.
 4. The method ofclaim 1, wherein the information related to the downlink data includesinformation related to lost protocol data units (PDUs).
 5. The method ofclaim 4, wherein the information related to the lost protocol data unit(PDUs) includes at least one of 1) sequence numbers of a PDCP PDUcorresponding to lost RLC PDUs, or 2) a highest PDCP PDU sequence numbersuccessfully delivered in sequence to the UE among PDCP PDUs transmittedto the source DU.
 6. The method of claim 1, wherein the UE context setuprequest message includes at least one of 1) a radio bearer (RB)identifier (ID), 2) a transport network layer (TNL) address for the CU,3) an uplink (UL) tunnel endpoint identifier (TEID) for the CU, 4) aradio link control (RLC) configuration, and/or 5) a logical channelconfiguration, per bearer.
 7. The method of claim 1, wherein the UEcontext setup request message includes an RRC context includinginformation related to at least one of 1) a random access channel (RACH)resource, 2) a RACH configuration, 3) a reference signal received power(RSRP), 4) a reference signal received quality (RSRQ), and/or 5) a beammeasurement for the UE.
 8. The method of claim 1, wherein the UE contextresponse message includes at least one of 1) a radio bearer (RB)identifier (ID), 2) a transport network layer (TNL) address for thetarget DU, and/or 3) a downlink (DL) tunneling endpoint ID (TEID) forthe target DU.
 9. The method of claim 1, wherein the UE context setupresponse message includes an RRC context including information relatedto at least one of 1) an allocated random access channel (RACH)resource, 2) a set RACH configuration, and/or 3) an allocated beam. 10.A central unit (CU) of a base station (BS) configured to operate in awireless communication system, the CU comprising: at least onetransceiver; at least processor; and at least one computer memoryoperably connectable to the at least one processor and storinginstructions that, based on being executed by the at least oneprocessor, perform operations comprising: receiving, from a sourcedistributed unit (DU) of the BS, a container including a measurementreport transmitted by a user equipment (UE); initiating mobility for theUE from the source DU to a target DU of the BS within the CU;transmitting, to the target DU, a UE context setup request message tocreate a UE context for the UE and allocate radio resources; receiving,from the target DU, a UE context response message in response to the UEcontext setup request message; generating a radio resource control (RRC)reconfiguration message to be provided to the UE; transmitting, to thesource DU, a message including information related to stopping datatransmission to the UE and the generated RRC reconfiguration message,wherein, after the source DU receives the message, data transmission tothe UE is stopped and the RRC reconfiguration message is provided to theUE; and receiving, from the source DU, information related to downlinkdata unsuccessfully transmitted to the UE, wherein RRC, packet dataconvergence protocol (PDCP) layers of the BS are located in the CU, andwherein radio link control (RLC), media access control (MAC), andphysical layers of the BS are located in the source DU and the targetDU.
 11. The CU of claim 10, wherein the message transmitted to thesource DU is a UE context modification request message.
 12. The CU ofclaim 10, wherein the information related to the downlink data isreceived via a downlink data delivery status frame.
 13. The CU of claim10, wherein the information related to the downlink data includesinformation related to lost protocol data units (PDUs).
 14. The CU ofclaim 13, wherein the information related to the lost protocol data unit(PDUs) includes at least one of 1) sequence numbers of a PDCP PDUcorresponding to lost RLC PDUs, or 2) a highest PDCP PDU sequence numbersuccessfully delivered in sequence to the UE among PDCP PDUs transmittedto the source DU.
 15. The CU of claim 10, wherein the UE context setuprequest message includes at least one of 1) a radio bearer (RB)identifier (ID), 2) a transport network layer (TNL) address for the CU,3) an uplink (UL) tunnel endpoint identifier (TEID) for the CU, 4) aradio link control (RLC) configuration, and/or 5) a logical channelconfiguration, per bearer.
 16. The CU of claim 10, wherein the UEcontext setup request message includes an RRC context includinginformation related to at least one of 1) a random access channel (RACH)resource, 2) a RACH configuration, 3) a reference signal received power(RSRP), 4) a reference signal received quality (RSRQ), and/or 5) a beammeasurement for the UE.
 17. The CU of claim 10, wherein the UE contextresponse message includes at least one of 1) a radio bearer (RB)identifier (ID), 2) a transport network layer (TNL) address for thetarget DU, and/or 3) a downlink (DL) tunneling endpoint ID (TEID) forthe target DU.
 18. The CU of claim 10, wherein the UE context setupresponse message includes an RRC context including information relatedto at least one of 1) an allocated random access channel (RACH)resource, 2) a set RACH configuration, and/or 3) an allocated beam. 19.A method performed by a user equipment (UE) configured to operate in awireless communication system, the method comprising: performing datatransmission with a source distributed unit (DU) of a base station (BS);transmitting, to the source DU, a measurement report, wherein mobilityfor the UE from the source DU to a target DU of the BS within a centralunit (CU) of the BS is initiated based on the measurement report;receiving, from the source DU, a radio resource control (RRC) messageafter the source DU receives from the CU a message including informationrelated to stopping data transmission to the UE and the RRCreconfiguration message, wherein the data transmission with the sourceDU is stopped after the source DU receives the message; disconnectingwith the source DU; transmitting, to the target DU, a RRCreconfiguration complete message; and performing data transmission withthe target DU, wherein RRC packet data convergence protocol (PDCP)layers of the BS are located in the CU, and wherein radio link control(RLC), media access control (MAC), and physical layers of the BS arelocated in the source DU and the target DU.